In a carburetor, a throttle valve for use with an internal combustion engine controls the volume of air-fuel mixture, for controlling the output of the engine. In an electronic fuel-injection system, such a throttle valve controls the amount of inhaled air, for controlling the output of the engine. The throttle valve is designed to interlock with the accelerator operation mechanism including the accelerator pedal.
In the past, the accelerator operation mechanism has been mechanically coupled to the throttle valve. In recent years, an apparatus has been proposed in which the throttle valve is opened or closed according to the position of the accelerator pedal by a driving means interlocking with a driving power source such as a motor. For example, Japanese Patent Laid-Open No. 145867/1980 discloses an apparatus in which a stepping motor is connected with the throttle valve and driven according to the position of the accelerator pedal.
Japanese Patent Laid-Open No. 153945/1984 recites countermeasures taken when the electronically controlled actuator driving the above-described stepping motor becomes uncontrollable. For instance, one countermeasure consists in disconnecting the throttle shaft from the electronically controlled actuator by a solenoid clutch and returning the throttle valve to its closed position by a return spring. In the prior art techniques, when the electronically controlled actuator no longer controls the motor, any driving means which opens or closes the throttle valve is not available. Therefore, the vehicle cannot be moved into a desired location for repair. In view of this drawback, a countermeasure has been proposed in Japanese Patent Laid-Open No. 153945/1980.
More specifically, a solenoid clutch is mounted between the throttle shaft and the rotating shaft that is rotated when the driver pushes down on the accelerator pedal. When the solenoid clutch is energized, it disconnects the throttle shaft from the rotating shaft. When the clutch is not energized, it connects the throttle shaft with the rotating shaft. A control circuit is provided to detect an abnormality in the control operation of the electronically controlled actuator. When such an abnormality is detected, the control circuit actuates a relay to deenergize the actuator and the solenoid clutch. If the actuator becomes uncontrollable, the throttle shaft is mechanically coupled to the accelerator pedal via the clutch.
In the technique described in Japanese Patent Laid-Open No. 153945/1984, uncontrollableness of the electronically controlled actuator is detected by the separate control circuit, which then deenergizes the actuator and the solenoid clutch. The above-cited Japanese patent specification further says that, when the actuator no longer controls the motor, the rotating shaft mechanically coupled to the accelerator pedal is coupled to the throttle shaft via the solenoid clutch. In the operation of the example described in the patent specification, when the electronically controlled actuator no longer controls the motor, the motor produces no driving torque. Therefore, when the driver presses down on the accelerator pedal, the throttle valve is opened or closed without hindrance. In this way. after the accelerator pedal has been pressed, the throttle valve is kept coupled to the actuator.
The solenoid clutch used in this prior art apparatus is large in size because of its structure. Also, the clutch is expensive to fabricate. Furthermore, the situation may get worse than the case in which the electronically controlled actuator becomes uncontrollable. That is, the possibility that the control circuit malfuntions cannot be totally excluded. For example, the throttle valve might be kept open by electromagnetic interference. In this case, even if a separate switching means is provided, and if the throttle shaft is connected to the accelerator pedal while deenergizing the solenoid clutch, then any means which causes the throttle valve to be closed against the action of the throttle shaft driven by the actuator does not exist. Therefore, it is difficult to secure a desired throttle opening. If the above-described circumstance occurs, it is the common practice for the driver to stop pressing down on the accelerator pedal and to push down on the brake pedal. In the conventional apparatus described above, the throttle valve is kept driven by the actuator.
In view of the foregoing problems, the present applicant has proposed an improved throttle control apparatus in Japanese patent application No. 22190/1989. In particular, if stoppage of operation of the accelerator pedal is detected, and if the present throttle opening is found to be in excess of the given opening, the driving means is separated from the throttle valve with certainty. The driving power source is stopped from controlling the throttle valve. The throttle valve can be directly driven within a given range of opening by operating the accelerator operation mechanism by more than a certain amount.
When a vehicle is traveling on a road covered with snow or other road surface having a low friction coefficient, if the vehicle is started or accelerated suddenly, then the drive wheels may spin due to excessive driving force, so that the directional stability of the vehicle is lost. To prevent this phenomenon, the traction is sometimes controlled to stop the drive wheels from spinning on acceleration. One specific known means for controlling the traction is to cut the supply of fuel to the engine as described in Japanese Patent Laid-Open No. 265428/1987. Another known means is to control the brakes and the throttle opening simultaneously. The former means for cutting the supply of fuel is installed on a vehicle equipped with an electronic fuel-injection system. The energization of the injector is controlled according to the condition of spin of the drive wheels by the electronic controller to limit the supply of fuel.
On the other hand, in the above-described throttle control apparatus that adjusts the throttle opening by the driving means such as a motor, the traction can also be controlled during control of the throttle valve by the driving means. Referring to FIG. 9(a), a reference speed Vs indicated by the dot-and-dash line has been set according to the speed of the driven wheels. The speed Vw of the drive wheels is indicated by the solid line. If the speed Vw of the drive wheels increases suddenly because they spin on acceleration, then the throttle opening is reduced rapidly to fully close the valve so that the speed Vw may approach the reference speed Vs. Hence, sufficient traction and lateral drag can be obtained. In FIG. 9, the phantom lines indicate the accelerator opening Ac that is determined by the amount by which accelerator operation mechanism is operated. The broken lines indicate the throttle opening Th.
In the above-described throttle control apparatus which can directly drive the throttle valve within the given range of opening, if the traction is controlled on acceleration to stop the drive wheels from spinning by the driving means in the same way as the foregoing, and if the driver operates the accelerator operation mechanism by more than a given amount to directly drive the throttle valve, then the throttle valve is not closed as shown in FIG. 9(b). For this reason, it is impossible to make the speed Vw of the drive wheels approach the reference speed Vs immediately. As a result, it takes long to stop the drive wheels from spinning.